1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to an apparatus that circulates and transfers a sheet having an image formed thereon so as to dry the sheet.
2. Description of the Related Art
In recent years, in an image forming apparatus (an ink-jet image forming apparatus) that forms an image on a sheet of paper by an ink-jet method, use of an aqueous ink containing a large amount of water therein has become the mainstream. In this regard, a higher proportion of water contained in an ink causes a sheet of ordinary paper to be likely to be curled to a larger extent when a print is made on the sheet, and may result in failures as follows. Specifically, jamming of sheets may occur during transfer of the sheets, or alignment of the sheets having been discharged may be deteriorated. In order to prevent such failures, it is necessary to employ such a measure as providing certain physical means for correcting the curling (an apparatus or the like that corrects the curling) to the image forming apparatus.
On the other hand, there is a known ink that is an aqueous ink but capable of reducing an occurrence of the curling of sheets, as described in Japanese Patent Application Publication No. 2005-220296. However, it is still inevitable that a sheet is curled due to the water content in the ink components on the sheet immediately after printing. Particularly in a high-speed ink-jet image forming apparatus, for example, with a print speed of 120 ppm (that is, capable of recording images on 120 sheets of paper in one minute), a time interval between two print sheets (a time interval from the time of printing a sheet to the time of printing the next sheet) is very short. For this reason, the curling of the sheet immediately after the printing is not negligible.
In view of the above problem, an image recording apparatus (an image forming apparatus) having the following configuration has been disclosed in Japanese Patent Application Publication No. 2006-264828. This image recording apparatus employs an ink-jet system, and includes a transfer belt that transfers a sheet during the image formation. In this image recording apparatus, the curling of a sheet immediately after printing is suppressed by changing a position at which a sheet having an image formed thereon is separated off the transfer belt. In the normal operation of the image recording apparatus described in Japanese Patent Application Publication No. 2006-264828, a sheet is separated off the transfer belt immediately after image formation on the sheet by ink-jetting is completed. However, in a case where a sheet which does not quickly dry is used, after image formation on the sheet by the ink-jet method is completed, the sheet is transferred as it is to a certain extent by the transfer belt, and thereafter is separated off the transfer belt.
The image recording apparatus described in Japanese Patent Application Publication No. 2006-264828 is capable of preventing the curling of a sheet immediately after an image is formed on the sheet. However, since the image recording apparatus requires additionally a mechanism for changing a position at which a sheet having an image formed thereon is separate off the transfer belt, there is a problem that the structure of the apparatus is complicated.